Beam scanning has become important for a variety of applications including scanned beam displays, bar code scanners, and electrophotographic printers. In prior art beam scanning applications, and especially in high performance applications, rotating polygon scanners have been in common use.
By way of example, the operation of a rotating polygon scanner in a beam scanning system will be described. FIG. 1 is a diagram illustrating the principal features of a typical rotating polygon-based beam scanning system 101. A laser diode 102 having a wavelength matched to the requirements of the application may optionally be modulated with an image data signal if required. Beam-forming optics 104 produce a laser beam having a desired shape and trajectory. The laser beam is reflected off a rotating polygon mirror 106, and particularly off the facets 108a, 108b thereof, individual facets 108a and 108b being indexed for clarity. It may be noted that the design of the beam scanning system 101 is such that the reflective surfaces 108a, 108b, etc. of the rotating polygon 106 are placed forward of the center of rotation such that the arriving beam sweeps over each mirror surface as the mirror surface is rotated. The beam is deflected across a deflection angle to form a scanned (and optionally, modulated) beam 110.
One difficulty encountered with rotating polygon-based beam scanning systems relates to the rotating polygon itself. Rotating polygon mirrors may suffer from relatively large mass, slow ramp-up to speed, large size, noise, bearing reliability issues, relatively high power consumption, and other shortcomings.
As mentioned above, high speed bar code scanners have typically used rotating polygon mirrors to produce scan rates and resolutions sufficient for throughput-sensitive applications such as in-counter scanning at retail check-out and high-speed package sorting. One example of such a scanner is described in U.S. Pat. No. 6,045,046 of Paul O. Detwiler, entitled FULL COVERAGE BARCODE SCANNER, hereby incorporated by reference. It may be noted that other such examples do not vary the angle of the polygon facets to create parallel scan paths, instead including in the scan path a vertical scan mirror that oscillates about the second axis.
As mentioned above, scanned beam displays also use beam scanners. Various scanned beam display embodiments have been disclosed in the art. One such example includes scanned beam displays such as that described in U.S. Pat. No. 5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, which is incorporated herein by reference. Similarly, electrophotographic printers (commonly referred to as “laser printers”), LIDAR systems, rotating laser levels, document scanners, and other beam scanning systems have heretofore made use of rotating polygon beam scanners. To a greater or lesser degree, these and other applications, when embodied using rotating polygon scanners, have suffered from the drawbacks inherent thereto.
In other applications, beam scanner performance may be conveniently characterized as the product of scan angle times mirror size, as a function of scan frequency. According to common usage, the product may be referred to as “theta-D” (ΘD) where theta refers to half the mechanical scan angle and “D” refers to mirror size. Implicit in this terminology is the assumption that static and dynamic deformation of the scan mirror surface remains within acceptable limits, frequently no more than ⅕ of the shortest wavelength of light being deflected (λ/5). Since a larger mirror size enables a smaller diffraction limited beam spot and a larger deflection angle allows a greater field width at a given distance over which to line up a row of spots, ΘD is proportional to the number of spots that may be resolved (e.g. displayed or detected) in a scan line. Frequency, of course, relates to the number of scan lines that may be produced per unit time. Hence, a larger ΘD scanner generally corresponds to higher performance.
According to the prior art, it has proven relatively difficult to achieve high ΘD at high scan frequencies while maintaining sufficient mirror flatness. Dynamic stresses on scan mirrors work against keeping mirrors flat when they are operated at combinations of relatively large scan angle, high frequency, and with a large mirror size.